1. Field of the Invention
This invention relates generally to a capstan motor drive control device that involves translating frequency signals into voltage signals and detecting the signal frequency and, more specifically, to a velocity detection device for translating frequency signals related to the speed of a moving object into voltage signals for detecting the moving velocity, as particularly applied to a tape drive system for a tape recorder.
2. Description of the Background
A typical tape drive system for a video tape recorder is shown in FIG. 10 and includes a capstan motor 20 consisting of a three-phase brushless DC motor. A driving circuit 21 causes three-phase currents proportional to an analog control voltage Vs to flow through the three u, v, and w phase coils to generate a torque in the capstan motor 20 and cause rotation of the capstan.
The rotational angular positions of the rotor (not shown) of the capstan motor 20 are detected by Hall devices 23. The position information as detected by these Hall devices 23 is fed back to the driving circuit 21 for changing over the currents flowing through the u, v, and w phase windings.
For detecting the angular velocity of the capstan motor 20, a so-called frequency generator FG made up of a magnet formed by magnetizing the outer periphery of a rotating part of the capstan motor and a magnetic reluctance (MR) sensor 24 arranged in proximity to the magnet is employed. The MR sensor 24 detects changes in the magnetic flux caused by the rotation of the magnetized part of the rotor and produces an analog signal having a frequency proportional to the angular velocity of the capstan motor 20. The frequency signals produced by MR sensor 24 are wave-shaped by a waveform shaping circuit 25 so as to be supplied as bistable square wave pulses FG to a micro-controller 22.
The micro-controller 22 functions as a frequency-voltage transducer that temporally measures the period of the FG pulse, based on its interrupt function of interruption with a rise timing of the FG pulse and an edge timing storage function. The micro-controller 22 detects the rotation velocity of the capstan motor 20 and feeds back velocity information to the drive circuit 21 to cause the tape to run at a constant velocity by way of speed servo control of the capstan motor 20.
The frequency-voltage transducer, or so-called FV converter, is widely employed as a frequency measurement unit for measuring the signal frequency translated into a corresponding voltage or as a velocity detection unit for detecting the rotational velocity of a motor, translated into an electrical voltage from frequency signals (FG pulses) proportional to the rotation velocity of the motor.
As shown in FIG. 11, the frequency-voltage transducer formed by the microcontroller 22 is made up of a period measurement unit 31 and a time/voltage converting unit 32. The period measurement unit 31 detects the time required for each period and the converting unit 32 converts a reciprocal of the detected period into an electrical voltage, which is the converted output Vs of FIG. 1.
Meanwhile, with the above-described frequency-voltage converter the reciprocal of the period of the frequency signals is directly translated into the corresponding electrical voltage, so that an instantaneous frequency of the signal cannot be detected and only the mean frequency may be detected with a delay equal to one period.
With the velocity detection unit employing such a frequency-voltage transducer, the tape running velocity as detected assumes discrete values and represents a mean velocity averaged over a period of the FG pulse signal. Consequently, with the conventional velocity detection unit, the calculated tape running velocity becomes less than the actual velocity during acceleration and greater than the actual velocity during deceleration.
In order to carry out control of acceleration and deceleration of the tape running speed with high accuracy in a tape drive system such as a video tape recorder, it is necessary first to correctly detect the tape running velocity. Nevertheless, with the conventional velocity detection unit as described above, only the mean velocity over a period of the FG pulse signal is obtained, so that the instantaneous velocity cannot be detected.
In general, a pinch roller for holding the tape in association with the capstan is provided in the tape drive system of the video tape recorder. The capstan is rotated at a constant velocity by a capstan motor, which is typically a DC motor rotating at a constant velocity. A voltage velocity signal indicating the rotation velocity of the capstan, that is, the tape running speed, is derived from the FG pulses that have a frequency proportional to the rotation velocity of the capstan motor. This speed signal is fed back to the driving circuit to cause the tape to run at a constant speed by way of servo control of the capstan motor velocity.
On the other hand, in such a tape drive system for a video tape recorder a capstan motor start-and-stop operation is repeated for stepwise feed during the low-speed playback mode. For this stepwise feed, a constant DC voltage of around +5 V is applied to start the capstan motor.
Meanwhile, if the constant voltage is applied to start the capstan motor, because the output torque of the capstan motor is constant, the starting characteristics of the capstan motor must change with changes in the motor load. Consequently, with the conventional video tape recorder the tape feed quantity by the stepwise feed for the low-speed playback mode changes, so that an identical head tracing cannot always be made. On the other hand, it has been necessary to cope with fluctuations in load or temperature characteristics from one version to another, or from one mechanical deck to another of the same model or version, by performing an operation of matching constants for a driving system of a capstan motor, such as the period during which the constant voltage is impressed.